1. Field of the Invention
The present invention relates to a multi-layer circuit board utilizing, as an insulating layer, a film made of a thermoplastic polymer capable of forming an optically anisotropic melt phase and also relates to a method of making such multi-layer circuit board.
In the description of the present invention, the thermoplastic polymer capable of forming an optically anisotropic melt phase will be referred to as a “thermoplastic liquid crystal polymer” unless otherwise specified and, also, the film utilizing such thermoplastic liquid crystal polymer will be referred to as a “thermoplastic liquid crystal polymer film” unless otherwise specified.
2. Description of the Related Art
In order to realize a three-dimensional electric connection in a multi-layer circuit board, a printed wiring board, which constitutes the multi-layer circuit board, includes wiring (often referred to as a “surface wiring” or a “circuit pattern”) formed on a surface of a printed wiring board, wiring (hereinafter referred to as an “intra-layer wiring”) formed so as to extend internally of a printed wiring board, and wiring (hereinafter referred to as an “interlayer wiring”) formed so as to extend completely across the thickness of some printed wiring boards.
The multi-layer circuit board generally makes use of sheets of materials having different chemical compositions and/or heat resistance properties such as melting points and is generally made by thermo compression bonding of one or more sheets having an intra-layer and/or a surface wiring and one or more sheets having no wiring formed therein. If the materials for the respective sheets forming the multi-layer circuit board have the same melting point, those sheets would melt simultaneously during the thermo compression bonding process. Therefore, the materials for the respective sheets must have melting points different from each other.
By way of example, the U.S. Pat. No. 5,719,354 and the Japanese Patent Application Laid-open No. Hei 11-309803 disclose a multi-layer circuit board comprising, as printed wiring boards, two films made of thermoplastic liquid crystal polymer and having surface wiring and intra-layer wiring, and, as a bonding layer, a thermoplastic liquid crystal polymer film interposed between the two films, wherein the bonding layer has a melting point lower than that of the two films. In this known multi-layer circuit board, the printed wiring boards are electrically insulated from each other by means of the bonding layer (See, for example, FIGS. 1D and 1E of the U.S. Pat. No. 5,719,354 and FIG. 1 of the Japanese Patent Application Laid-open No. Hei 11-309803). Specifically, the U.S. Pat. No. 5,719,354 describes, in column 5, lines 61 to 64 and column 6, lines 4 to 6 thereof, that drilling to form perforations through the bonding layer and a succeeding metal application are necessary in order to realize the electric connection between the printed wiring boards.
For the drilling discussed above, a drill or a laser have been conventionally employed, in which a drilling process of drilling predetermined sites to form respective holes is conducted. And for the metal application discussed above, one method (hereinafter referred to as a “plating method”) has been conventionally employed, in which chemical and electric plating processes to form plated layers over the surfaces of the wall in the holes are conducted in succession. In addition, another method (hereinafter referred to as a “pasting method”) can be employed for that purpose, in which an electroconductive paste is applied, or otherwise printed, to fill the hole formed by means of a laser or by the use of a drill and is subsequently cured.
It has, however, been found that the multi-layer circuit board that requires formation of holes extending through the printed wiring board and bonding layers has a problem in that neither formation of wiring nor mounting of electronic parts can be effected onto front and rear surface regions of the circuit board where holes for interlayer connection are formed. Accordingly, not only is this known multi-layer circuit board incapable of meeting with the recent demand for a high-density surface mounting of electronic parts, but the method of making the known multi-layer circuit board includes a series of manufacturing steps, some of which appear to be redundant enough to require a complicated process control.